@tintenfuchs - So I used to work at a bakery as a cookie decorator...
@AnasaziWrites has it correct that you need to make sure you're using an edible material - so royal icing is the go-to.
Even with royal icing and a very small piping tip, the size you're looking at is VERY SMALL! I'd actually try to use something that resembles a henna cone to get detail that small. But also, are you good at piping? because piping that small is difficult.
If piping is not going to work for you - you can glaze the cookies and then write on them with edible markers or food coloring pens. You could write with them on the unglazed cookies, but it won't look very sharp.
Why must these be done "on site"? - that's quite unusual! Usually we handle all the decoration at the bakery, because it's a "clean kitchen" that is maintained to industry standards for hygiene and safety.
--best wishes with the project! K